Method for producing sintered electroceramic materials from hydroxide and oxalate precursors

ABSTRACT

A method for producing electroceramic materials of high sintered density from hydroxide and/or oxalate precursors is disclosed. In this method the precursor(s) are compacted to form a preform, thermally treated, recompacted and then sintered to form finished products.

International application PCT/SG99/00056 was published under PCT Article21(2) in English. This application is the national phase ofinternational application PCT/SG99/00056 filed Jun. 4, 1999whichdesignated the U.S.

FIELD OF THE INVENTION

The present invention relates to the fabrication of electroceramicmaterials, such as lead zirconate titanate and barium strontiumtitanate, of high sintered density at a lowered sintering temperature.More particularly, the invention advantageously provides a method forthe production of electroceramics of high density from coprecipitatedhydroxides and oxalates without the use of any sintering aids/additives.The direct processing technique also advantageously offers a uniqueadvantage in minimizing the level of contamination in the sinteredelectroceramics, due to the elimination of certain intermediateprocessing steps, such as the calcination and subsequent ball milling ofprecursor powders.

BACKGROUND OF THE INVENTION

Electroceramics, such as lead zirconate titanate (PZT), lead lanthanumzirconate titanate (PLZT) and barium strontium titanate (BST), aretechnologically important materials in electronics and microelectronicsdue to their unique piezoelectric, ferroelectric and many otherelectromechanical, pyroelectric and optoelectrical properties. On theone hand, high sintered density and uniform microstructure are among themost desirable features for almost all the electroceramics in achievingmany of these desirable electrical properties. It is however difficultto achieve a sintered density close to the theoretical density for mostof the electroceramic materials via conventional ceramic processingroutes using mixed oxides as the starting materials. This, together withmany of the undesirable features of sintered electroceramics, such asnon-stoichiometry and wide fluctuations in composition andmicrostructure, are to a large extent due to inadequate processingroutes chosen for fabricating these materials. For example, as a resultof the high volatility of PbO at elevated temperatures, there is asignificant loss of lead oxide in the production of lead-containingelectroceramics such as PZT, PLZT and lead magnesium niobate (PMN) athigh sintering temperatures of >1200° C., leading to the formation ofone or more pyrochlore phases and therefore a reduction in sintereddensity. On the other hand, the use of electroceramic materials inco-firable mulitlayer electronic and microelectronic devices for manyelectromechanical applications requires that the electroceramiccompositions be sinterable at temperatures below 1000° C., as a loweredfiring temperature will apparently alleviate, if not completelyeliminate, the detrimental interactions between the ceramic layer andthe electrode layer seen at a higher sintering temperature. It isenvisaged that a reduction in the sintering temperature ofelectroceramic materials will eventually lead to the substitution ofvery expensive electrodes, such as platinum and palladium, by muchcheaper ones such as silver, nickel, copper and their alloys. Therefore,there is considered to be substantial technological and economicsignificance in methods of lowering the sintering temperature ofelectroceramics without sacrificing the electrical properties thereoftoo greatly.

Two general approaches have been taken in order to lower the sinteringtemperature of electroceramics, preferably to the range of <1000° C. forPZT as an example. These include (i) employing an ultrafine startingpowder prepared mainly via various chemistry-based processing routes;and (ii) using a sintering aid/additive of low melting point, such asV₂O₅, Bi₂O₃, and an eutectic mixture of CuO and barium or strontiumoxide, as claimed by Buchanan and Wittmer in U.S. Pat. No. 4,283,228(1981) and Srivastava, Bhalla and Cross in U.S. Pat. No. 5,433,917(1995), respectively. Unfortunately, many of the chemistry-based powderpreparation routes for electroceramics are associated with disadvantagessuch as very high manufacture cost and low production yield, which makethem unsuitable for industrial scale production of electroceramiccomponents. Furthermore, most of these have yet to demonstrate anysignificant advantages in lowering the sintering temperature ofelectroceramic materials over the conventional electroceramic processingroutes. The approach of employing sintering aids/additives is attractivein terms of being able to lower the sintering temperature ofelectroceramics by forming a liquid phase at the grain boundaries andgrain junctions, as has been demonstrated for PZT to below 1000° C.However, the sintering aids/additives are often detrimental to theelectrical properties of the electroceramics, due to the formation of asecondary non-electroceramic phase concentrated at the grain boundariesand grain junction of sintered electroceramics. In addition to this, themajority of the sintering aids/additives suggested are extremely toxicand therefore are very difficult to handle in any large scale productionof electroceramic components.

Using high-purity inorganic or organic salts as the starting materials,precursor-calcination-milling-pelleting-sintering is the wellestablished fabrication route for many electroceramics. The precursor isnormally prepared via a wet-chemistry route, such as sol-gel,hydrolysis, hydrothermal reaction or coprecipitation, followed bycalcination and ball milling steps in order to form the requiredelectroceramic phase and to adjust the powder characteristics. Thecalcination of precursor powders at an intermediate temperatureunfortunately removes almost all the advantages offered by most of thechemistry-based powder preparation techniques, including very highspecific surface area, ultrafine particle size and narrow particle sizedistribution. This is a result of particle agglomeration in the calcinedelectroceramic powders. The presence of hard particle agglomeratesadversely affects subsequent compaction and sintering behaviour ofceramic powders and results in a reduced density and the occurrence ofmicrostructural defects in sintered electroceramics, as observed by F.F. Lange, see J. Amer. Ceram. Soc., 66, pp.396-398 (1983) and W. H.Rhodes, See J. Amer. Ceram. Soc., 64, pp. 19-22 (1981). Apost-calcination milling process is generally required in order tomodify the powder characteristics, e.g., the particle/agglomerate sizeand particle morphology, before the powder is shaped to a powder compactor a component shape and then densified at high sintering temperatures.However, some hard particle agglomerates (aggregates) can not beeffectively eliminated by conventional milling such as ball milling.Furthermore, a mechanical milling inevitably introduces contaminationinto the electroceramic materials. Contamination in the range of 0.1 to2 wt % is common under normal milling conditions as pointed out byMoulson and Herbert (Electroceramics, Chapman and Hall, London, 1990).Such high levels of contamination are unacceptable for manyelectroceramic materials.

Reaction sintering, in which the reaction between constituent componentphases occurs concurrently with the densification process at thesintering temperature, has been established as a viable fabricationtechnique for a wide range of oxide and non-oxide ceramics. It offers atleast two advantages over the conventional ceramic processing routes:(i) no intermediate milling and drying of pre-reacted and post-reactedcompounds are required; and (ii) the free energy associated with thereaction helps facilitate densification. For example, T. R. Shrout, P.Papet, S. Kim, and G. Lee, see J. Amer. Ceram. Soc., 73, pp.1862-1867(1990), and S. Kim, G. Lee, T. R. Shrout and S.Venktanari, see J. Mater.Soc., 26, pp.4411-4415 (1991), observed that the densification of PZTwas enhanced by the reaction of constituent oxides in a partiallyreacted system. Since the precursors exhibit a higher degree of mixinghomogeneity, the reaction may be completed at a lower temperature thanthose for mixed oxides.

SUMMARY OF THE INVENTION

The present invention relates to the fabrication of electroceramicmaterials of high sintered density at sintering temperatures ofsubstantially lower than those normally required by the traditionalprecursor-calcination-milling-pelleting-sintering route, without use ofany sintering aids/additives.

Accordingly, the present invention provides a method for producing anelectroceramic material from hydroxide and/or oxalate precursors, saidmethod comprising:

compacting a mixture containing at least one hydroxide and/or oxalateprecursor to form a preform;

thermally treating the preform at a predetermined temperature;

re-compacting the preform; and

sintering the re-compacted preform to form said electroceramic material.

The invention also provides an electroceramic material produced by themethod of the immediately preceding paragraph.

In a preferred embodiment, the invention involves the use of fine andreactive hydroxide and oxalate precursors, preferably prepared viacoprecipitation from aqueous solutions containing desirable cations. Theprecursors are not calcined at an intermediate temperature in order toavoid the adverse effects of particle agglomeration on densification ofelectroceramics at the sintering temperature. Instead, they are directlyshaped into pellets or any other complex shapes by compaction. Thepellets or preforms of hydroxide or oxalate precursors are thenthermally treated in a controlled manner preferably at a temperature inthe range of 400° C. to 700° C., followed by a re-compaction, preferablyby isostatic pressing to further increase their green densities.Sintering of the isostatically pressed compacts is carried out at atemperature which may advantageously be ˜200° C. lower than thatrequired in traditionalprecursor-calcination-milling-pelleting-sintering routes

Compared with conventionalprecursor-calcination-milling-pelleting-sintering route, theintermediate calcination and milling steps of precursor powders areadvantageously eliminated and a lower sintering temperature mayadvantageously be used in the present method. Using the present method,hydroxide-derived lead zirconate titanate (PZT) may be sintered to arelative density of >98% theoretical at temperatures below 1000° C.without the use of any sintering aids/additives. Similarly theoxalate-derived barium strontium titanate (BST) may be sintered to adensity of >99% theoretical density at a temperature of 1200° C. for 1hour. These sintering temperatures are, as discussed above substantiallylower than those required by conventionalprecursor-calcination-milling-pelleting-sintering routes.

DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inmore detail with reference to the accompanying drawings, in which:

FIG. 1 is a flow diagram summarizing the various steps involved in thedirect processing technique of electroceramics from hydroxide andoxalate precursors. Intermediate steps, such as the calcination ofprecursor powders and subsequent milling of the calcined electroceramicphases, are omitted, leading to a lowering in the sintering temperaturesof, for example, PZT and BST.

FIG. 2 is an XRD pattern of a Pb(Zr_(0.52)Ti_(0.48))O₃ sintered at 1050°C. for 1.0 hour prepared according to the invention.

FIG. 3 is an XRD pattern of a Br_(0.65)Sr_(0.35)TiO₃ sintered at 1200°C. for 1.0 hour prepared according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A direct processing method for producing electroceramic materials, suchas PZT and BST of various compositions and PLZT, from hydroxide andoxalate precursors is iprovided which advantageously enables thefabrication of highly sintered electroceramics at a sinteringtemperature of ˜200° C. lower than those normally required bytraditional precursor-calcination-milling-pelleting-sintering routes.The lowered processing temperatures for electroceramics do not involvethe use of any sintering aids/additives and therefore the electricalproperties of the electroceramics are not adversely affected thereby.

The starting materials used in the direct processing technique arepreferably commercially available inorganic chemicals, such as leadnitrate, zirconium oxynitrate, titanium tetrachloride, strontium nitrateand barium nitrate, in either solid or solution forms. From theseinorganic chemicals an aqueous solution containing required cations suchas Pb²⁺, Zr⁴⁺, Ti⁴⁺ Ba²⁺ and Sr²⁺, at a designed molar ratio isprepared.

Sintering of the re-compacted preform is carried out at a temperature,generally determined as a function of the electroceramic material beingproduced. Preferably, the sintering temperature is in the range of from90° C. to 1400° C.

As already discussed, the electroceramic material may include, forexample PZT or BST. In a preferred embodiment, the electroceramicmaterial comprises PZT having the formula Pb(ZrTi)O₃ with a Zr:Ti molarratio ranging from 80:20 to 20:80, more preferably 65:35 to 35:65.

According to one embodiment the coprecipitation of Pb(ZrTi)O₃ hydroxidesis carried out by slowly adding an aqueous solution containing Pb²⁺,Zr⁴⁺, Ti⁴⁺ cations to an ammonia solution of ˜pH9. The resultingcoprecipitates are then preferably aged for 1.0 hour in the supematantliquid before being recovered by filtration and dried at a temperatureranging from 70° C. to 120° C. for 2 hours in an oven. To fabricatesintered PZT ceramics, the as-dried precursor powders are preferablycompacted uniaxially in a hardened steel die at a pressure in the rangeof 20 to 100 MPa. Thermal treatment of the precursor pellets ispreferably then carried out for 4 hours at a temperature in the rangefrom 400° C. to 700° C., using a heating rate of 2° C./minute. Thethermally treated powder compacts are further isostatically pressed,preferably at a pressure in the range of 200 to 500 MPa. Sintering ofthe isostatically pressed powder pellets is made in air at a temperaturein the range of 950° C. to 1200° C. for a duration of 1 to 4 hours, withboth heating and cooling rates being fixed at 5° C./minute. Thehydroxide-derived lead zirconate titanate, Pb(Zr_(0.52)Ti_(0.48))O₃, issintered to a relative density of >98% theoretical at 980° C. for 4hours without the use of any sintering aids/additives.

According to another embodiment the direct processing technique has alsobeen used to fabricate sintered Ba_(1-x)Sr_(x)TiO₃. For this, an aqueoussolution containing Ba²⁺, Sr²⁺ and Ti⁴⁺ cations at a designed molarratio is prepared. An appropriate amount of aqueous solution containing30 wt % C₂H₂O₄ is then slowly titrated into the aqueous solutioncontaining Ba²⁺, Sr²⁺ and Ti⁴⁺ at a rate of 5 ml/minute, resulting inthe coprecipitation of Ba-Sr-Ti-oxalates. To remove the Cl⁻, the oxalategels are filtered and washed repeatedly using de-ionized water. They arethen aged for 2 hours in the supematant liquid before being recovered byfiltration and subsequently dried at 70° C. to 120° C. for 10 hours inan oven. To fabricate sintered BST, the as-dried precursor powder wascompacted uniaxially in a hardened die at a pressure in the range of 20to 100 MPa. The pellets were then heated at a rate of 1° C./minute to atemperature in the range of 500° C. to 700° C., where they are kept for10 hours, followed by an isostatic pressing at a pressure in the rangeof 200 to 500 MPa. Sintering of the isostatically pressed pellets isthen carried out at various temperatures in the range of 1000° C. to1400° C., more preferably 1100° C. to 1250° C., while both the heatingand cooling rates were fixed at 5° C./minute. In this embodiment,oxalate-derived barium strontium titanate (Ba_(0.65)Sr_(0.35)TiO₃) maybe sintered to a density of >99% theoretical at 1200° C. for 1 hour.

Preferred embodiments of the present invention will be furtherdemonstrated by the following examples. These examples illustrate thefabrication of PZT ceramics of different Zr:Ti ratios and BST ceramics,without the use of any sintering aids/additives. These examples are notintended to limit the scope of this invention.

EXAMPLE 1 DIRECT PROCESSING OF Pb(Zr_(0.52)Ti_(0.48))O₃ FROM HYDROXIDEPRECURSORS

The starting materials were commercially available Pb(NO₃)₂, TiCl₄ andZrO(NO₃)₂ (e.g. in the form of an aqueous solution containing 20 wt %ZrO₂) of high purity. To prepare an aqueous solution containing Pb²⁺,Zr⁴⁺ and Ti⁴⁺ cations in the molar ratio of 1:0.52:0.48, an appropriateamount of chilled (˜4° C.) de-ionized water was slowly added into a coolTiCl₄ solution while being stirred (TiCl₄:water ratio: 1:14.25). Apre-weighted ZrO(NO₃)₂ solution (as required for the Zr:Ti ratio in PZT)was then blended into the TiCl₄-water mixture, before a desirable amountof cold ammonia solution (12 wt %) was added into the mixed solution toobtain a pH level of ˜10 for the mixed solution. The addition of ammoniasolution resulted in the coprecipitation of Zr-Ti-hydroxides. To removethe Cl⁻ ions, the gelatinous Zr-Ti-coprecipitate was filtered and washedrepeatedly using de-ionized water until the pH of filtrate was close to7.0 and no trace of Cl⁻ could be detected using AgNO₃. An aqueousoxynitrate solution containing Zr⁴⁺ and Ti⁴⁺ cations equivalent to 7 wt% ZT was subsequently prepared by dissolving the white coprecipitateinto an appropriate amount of 3.0M HNO₃. The desirable amount ofPb(NO₃)₂ as required for the Pb:Zr:Ti ratio in PZT was first dissolvedin de-ionized water and was then combined into the Zr-Ti-oxynitratesolution, for the formation of designed nitrate solution containingPb²⁺, Zr⁴⁺ and Ti⁴⁺ cations in the molar ratio of 1:0.52:0.48 (PZTconcentration: 12 wt %).

The coprecipitation of Pb(ZrTi)O₃ hydroxides was carried out by slowlyadding the mixed nitrate solution into an ammonia solution of pH9, whichwas checked and maintained by adding an appropriate amount ofconcentrated ammonia solution during the coprecipitation process. Thecoprecipitates were then aged for 1 hour in supernatant liquid beforebeing recovered by filtration and dried at 90° C. for 2 hours in anoven. To fabricate sintered PZT ceramics, the as-dried precursor powderswere compacted uniaxially in a hardened steel die of 12.5 mm in diameterat a pressure of 50 MPa. Thermal treatment of the precursor pellets wascarried out for 4 hours at a temperature in the range from 400 to 700°C., using a heating rate of 2° C./minute. This resulted in a weight lossin the range of 20 to 25%, depending on the thermal treatmenttemperature. The total weight loss with respect to the startingmaterials was in the range of 40 to 45%. The thermally treated powdercompacts were further isostatically pressed at a pressure in the rangeof 200 to 500 MPa, resulting in a green density in the range of 60 to70% theoretical. A one-dimensional shrinkage in the range of 8 to 10%was observed for the powder pellets when they were subjected to anisostatic pressing at 350 MPa. The resulting green density was 5.60,5.10, and 5.05 g/cm³, equivalent to 70.0, 63.8 and 63.2% PZT theoreticaldensity, for the pellets thermally treated at 400, 500 and 600° C.,respectively, as measured on the basis of pellet mass and dimensions.Sintering of the isostatically pressed powder compacts was made in airat a temperature in the range of 950 to 1 200° C. for a duration of 1 to4 hours, with both heating and cooling rates being fixed at 5°C./minute.

The Pb(Zr_(0.52)Ti_(0.48))O₃ pellets thermally treated at 500° C. for 4hours and then isostatically pressed at 350 MPa exhibited a sintereddensity of 7.37, 7.70, 7.93, 7.81 and 7.74 g/cm³ as measured usingimmersion technique in de-ionized water, when sintered for 1 hour at950, 1000, 1050, 1100 and 1150° C., respectively. Their dimensionalshrinkages were in the range of 25 to 30%, depending on the sinteringtemperature. This suggests that the sintered density increases withrising temperature over the range from 950 to 1050° C., where itmaximizes. Further increasing the sintering temperature above 1050° C.results in a slight fall in sintered density, presumably due to the lossof lead oxide together with the occurrence of exaggerated grain growthat too high a temperature. It is believed that the evaporation of leadoxide at the sintering temperature will be significantly reduced byembedding the PZT pellets in an appropriate PZT powder. The sinteringtemperature (1050° C.), at which a maximum sintered density of 99.2%theoretical density is achieved, is considerably lower than those (inthe range of ˜1300° C.) generally required by powders prepared via theconventional solid state reaction and many chemistry-based processingroutes. At the same time, the sintered density of PZT via the directprocessing technique is among the highest ever achieved by pressurelesssintering. An average grain size of ˜10 m was measured, using the lineinterception technique, for the PZT with a sintered density of 99.2%theoretical density. It was yellow in appearance and exhibited a roomtemperature dielectric constant of 1024 and a dielectric loss of 2.1%when measured using a HP 4284A LCR meter at a frequency of 1.0 kHz. FIG.2 is a XRD trace for Pb(Zr_(0.52)Ti_(0.48))O₃ sintered at 1050° C. for 1hour, indicating that it is of high purity.

The Pb(Zr_(0.52)Ti_(0.48))O₃ composition treated at 600° C. for 4 hoursexhibited a sintered density of 7.22, 7.56, 7.78, 7.63 and 7.54 g/cm³when sintered for 1 hour at 950, 1 5 1000, 1050, 1100 and 1150° C.,respectively. They were lower than those of Pb(Zr_(0.52)Ti_(0.48))O₃pellets treated at 500° C. at each of the sintering temperatures. Forthose thermally treated at 400° C. and 700° C., the resultant sintereddensities are poorer.

To further investigate the effect of sintering time at temperaturesbelow 1000° C., the pellets thermally treated at 500° C. were sinteredfor 4 hours at various temperatures ranging from 900 to 990° C. ThePb(Zr_(0.52)Ti_(0.48))O₃ sintered at 950° C. exhibited a sintereddensity of 7.76 g/cm³ and had a room temperature dielectric constant of956 and a dielectric loss of 2.6% at a frequency of 1.0 kHz. Thespecimen sintered at 980° C. for 4 hours exhibited a sintered density of98.1% theoretical density and registered a room temperature dielectricconstant of 975 and a dielectric loss of 2.4%. These values arecomparable to those reported for conventional PZT sintered attemperatures in the range of 1200 to 1300° C., demonstrating theeffectiveness of the direct processing technique in lowering thesintering temperature of PZT derived from hydroxide precursors, withoutthe use of any sintering aids/additives.

In summary of the above example, inexpensive inorganic chemicals, suchas Pb(NO₃)₂, TiCl₄ and ZrO(NO₃)₂, are used as the starting materials forthe fabrication of highly densified Pb(Zr_(0.52)Ti_(0.48))O₃ at atemperature much lower than that normally required in the traditionalprecursor-calcination-milling-pelleting-sintering route. The noveldirect processing route does not involve the intermediate calcinationand milling steps of precursor powders and the use of sinteringadditives.

EXAMPLE 2 DIRECT PROCESSING OF Pb(Zr_(0.65)Ti_(0.35))O₃ FROM HYDROXIDEPRECURSORS

A coprecipitated PZT hydroxide composition with the Pb:Zr:Ti molar ratioof 1.0:0.65:0.35 was prepared from Pb(NO₃)₂, TiCl₄ and ZrO(NO₃)₂ (anaqueous solution containing 20 wt % ZrO₂) by following the proceduredetailed above. The powder pellet of hydroxide precursor was thermallytreated at 500° C. for 4 hours and then isostatically pressed at 350MPa. The thermal treatment resulted in a weight loss of 24.2%. Thepellet was subsequently sintered at 1050° C. for 4 hours, resulting in asintering shrinkage of 26.7% and a sintered density of 7.74 g/cm³. Thesintered Pb(Zr_(0.65)Ti_(0.35))O₃ appeared light yellow and exhibited aroom temperature dielectric constant of 603 and a room temperaturedielectric loss 3.6%. The composition was also sintered for 1 hour at1050, 1100, 1150 and 1200° C., respectively, resulting in a respectivesintered density of 7.38, 7.48, 7.78 and 7.82 g/cm³. The sintered PZTexhibited a dielectric constant of 463, 502, 607 and 643 and adielectric loss of 6.1, 5.0, 3.4 and 2.5%, respectively, for thesesintering temperatures.

EXAMPLE 3 DIRECT PROCESSING OF Pb(Zr_(0.35)Ti_(0.65))O₃ FROM HYDROXIDEPRECURSORS

A coprecipitated PZT composition with the Pb:Zr:Ti ratio of1.0:0.35:0.65 was prepared from Pb(NO₃)₂, TiCl₄ and ZrO(NO₃)₂ (anaqueous solution containing 20 wt % ZrO₂) by following the proceduredetailed above. The thermal treatment and isostatic pressing scheduleswere the same as in example 2 for the composition ofPb(Zr_(0.65)Ti_(0.35))O₃. A weight loss of 22.7% was observed when thepellet was thermally treated at 500° C. for 4 hours. The sinteredPb(Zr_(0.35)Ti_(0.65))O₃ was heavy yellow in appearance and showed asintered density of 7.48 g/cm³ when sintered at 950° C. for 4 hours. Itsroom temperature dielectric constant and room temperature dielectricloss were 517 and 2.7%, respectively, at a frequency of 1.0 kHz. The PZTcomposition was also sintered for 1 hour at 950, 1000,1050 and 1100° C.,respectively, leading to a respective sintered density of 7.71, 7.77,7.90 and 7.82 g/cm³. Their room temperature dielectric constant and roomtemperature dielectric loss were 480, 491, 532 and 514, and 3.9, 3.4,2.2 and 2.9%, respectively, for these sintering temperatures.

EXAMPLE 4 DIRECT PROCESSING OF Ba_(0.65)Sr_(0.35)TiO₃ FROM OXALATEPRECURSORS

The starting materials for BST were commercially available bariumnitrate, Ba(NO₃)₂, strontium nitrate, Sr(NO₃)₂, titanium tetrachloride,TiCl₄ and oxalate acid, C₂H₂O₄2H₂O of high purity. To prepare an aqueoussolution containing Ba²⁺, Sr²⁺ and Ti⁴⁺ cations of the molar ratio of0.65:0.35:1.0 (BST concentration: 10 wt %), an appropriate amount ofchilled (˜4° C.) de-ionized water was slowly added into a cool TiCl₄solution while being stirred (TiCl₄:water ratio: 1:15). The desirableamounts of Ba(NO₃)₂, and Sr(NO₃)₂ as required for the Ba:Sr:Ti ratio inBST were first dissolved in de-ionized water and then combined into theTiCl₄-water solution. A pre-weighted amount of C₂H₂O₄2H₂O, as worked outon the basis of the C₂H₂O₄2H₂O:BST molar ratio: 3.5:1.0 was dissolved inde-ionized water to prepare an aqueous solution containing 30 wt %C₂H₂O₄. The oxalic acid solution was then slowly titrated into theaqueous solution containing Ba²⁺, Sr²⁺ and Ti⁴⁺ at a rate of 5ml/minute, resulting in the co-precipitation of Ba-Sr-Ti-oxalates. Toremove the Cl⁻, the oxalate gels were filtered and washed repeatedlyusing de-ionized water until no trace of Cl⁻ could be detected usingAgNO₃. They were then aged for 2 hours in the supernatant liquid beforebeing recovered by filtration and subsequently dried at 100° C. for 10hours in an oven. To fabricate sintered BST, the as-dried precursorpowder was compacted uniaxially in a hardened die of 12.5 mm in diameterat a pressure of 30 MPa. The pellets were then heated at a rate of 1°C./minute to 600° C., where they were kept for 10 hours, followed by anisostatic pressing at a pressure of 350 MPa. A weight loss of 50.8% wasnoted when the pellet was subjected to the thermal treatment. Sinteringwas then carried out at various temperatures in the range of 1100 to1250° C. for 1 hour, while both the heating and cooling rates were fixedat 5° C./minute. The sintering shrinkages were in the range of 40 to45%, depending on the sintering temperature. A sintered density of 5.15,5.50, 5.70 and 5.61 g/cm³ was measured for the pellets sintered for 1hour at 1100, 1150, 1200 and 1250° C., respectively, as determined usingimmersion technique in de-ionized water. The sintered BST ceramic at1200° C. for 1 hour, which was grey in appearance and exhibited arelative density of 99.4% theoretical density, demonstrated a dielectricconstant of 9000, a dielectric loss of 0.23% and a curie temperature of˜20° C. at a frequency of 1 MHz. As shown by the XRD trace in FIG. 3,the directly processed Ba_(0.65)Sr_(0.35)TiO₃ is of considerably highpurity.

What is claimed is:
 1. A method for producing an electroceramic materialselected from the group consisting of lead zirconate titanate (PZT) andbarium strontium titanate (BST), said method comprising: (a) compactinga precursor of said electroceramic material to form a preform, saidprecursor selected from the group consisting of hydroxides and oxalatesof said electroceramic material; (b) thermally treating the preform at apredetermined temperature; (c) re-compacting the preform; and (d)sintering the re-compacted preform to form said electroceramic material.2. A method according to claim 1, wherein the preform is thermallytreated at a temperature in the range of 400° C. to 600° C.
 3. A methodaccording to claim 1, wherein the re-compaction of the preform iscarried out in a cold isostatic press.
 4. A method according to claim 1,wherein sintering of the recompacted preform is carried out at atemperature in the range of 900° C. to 1400° C.
 5. A method according toclaim 1, wherein Pb(Zr_(0.52)Ti_(0.48))O₃ is produced from hydroxideprecursors, and wherein the sintering of the re-compacted preform iscarried out at a temperature in the range of 900° C. to 1100° C.
 6. Amethod according to claim 1, wherein barium strontium titanate isproduced from a precursor of oxalates, and wherein the sintering of there-compacted preform is carried out at a temperature in the range of1000° C. to 1400° C.
 7. A method according to claim 6, whereinBa_(0.65)Sr_(0.35)TiO₃ is produced from oxalate precursors, and whereinsintering is carried out at about 1200° C., the relative density of theBa_(0.65)Sr_(0.35)TiO₃ being about 99.4% of theoretical density.
 8. Amethod according to claim 1, wherein said lead zirconate titanate hasthe formula Pb(ZrTi)O₃ with Zr:Ti molar ratio ranging from 80:20 to20:80.
 9. A method according to claim 1, additionally comprising formingsaid hydroxide and/or oxalate precursors by coprecipitation of anaqueous solution of preselected cations.